Animal Diversity Richness & Animal Rarity

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Animal Diversity Richness

Richness maps of mammals, birds, amphibians, and reptiles help in assessing the overall health of the environment. Since richness represents the number of species found in an area, it tends to be high in places where urbanization is low. Biologists compile animal richness data from field studies using manual counts of species in the wild. Over time, many
studies by hundreds of biologists produce a large store of information on the world’s species that go into making accurate maps.
 
Animal richness calculations can be done for entire continents, states, or even one’s backyard. A simple species richness calculation using some of Michigan’s native animals as examples is shown in the following table. In this example, areas 2 and 4 have little species richness, but areas 3 and 5 contain rich animal diversity.

Animal Rarity

Rare animals, like rare plants, are not necessarily endangered, because every animal is rare somewhere in the world. Migration complicates anixvi

 

mal rarity because there may be times when a wetland is almost covered bill-to-tail with snow geese, yet almost on cue, thousands of geese rise into the air to begin their migration. Within a week the wetland that held uncountable numbers of geese now contains one or two, or none.

In addition to natural migrations, some animals roam over expansive ranges, and so rarity calculations must account for this movement. Consider a species (species A) that occurs in four of five study areas; it therefore has a large range. A second species (species B) may have been spotted in only two of five areas; it has a smaller range. Range-size rarity is the inverse of the number of areas where a species has been counted. For the two species in this example, range-size rarity values are as follows:

Species A range-size rarity = 1/4 = 0.25
Species B range-size rarity = 1/2 = 0.5

For individual species, a higher index number signifies greater rarity. The range-size rarity for the single-study area above equals 0.75. Range-size rarities for more than one hypothetical study area are illustrated in the two following tables.

In calculating an area’s range-size rarity, rather than single species, higher values indicate areas with greater diversity (less rarity). The rangesize rarity calculation therefore accounts for the range of an animal in addition to its abundance.

Animal migrations and movements over a range contribute to an animal’s RWRI. Rarity-weighted richness values become most useful when compared over a period of time to determine whether a species is growing more abundant or slipping toward extinction. It should not be difficult to imagine the rarity indexes (if they had been calculated) of American bison on the Great Plains in 1775, 1830, and 1995. In other words, as the years
progressed, the bison’s RWRI increased.